In order to be able to correct external factors affecting combustion quality, such as variations in fuel quality, temperature or pressure fluctuations, the ratio of air to fuel, the so-called air ratio or lambda λ, must be adjusted. A corresponding setup is also known as a fuel/air interconnection. A particularly inexpensive sensor for measuring lambda is the ionization electrode. When an AC voltage is applied, there flows through the electrode and flame an ionization current which is adjusted to a setpoint value specified as a function of the respective output of the burner. Using such an arrangement, the air ratio can be controlled, as the ionization current is a function of the air ratio at the respective output level. The AC voltage is adjusted to a voltage setpoint by a voltage regulator.
A signal processing arrangement for a burner system of the type mentioned in the introduction is indicated in DE-C2-19632983. This publication mentions a fuel/air connection having a signal detection circuit according to DE-A1-4433425 wherein an additional compensation circuit for the AC voltage applied to the ionization electrode is apparently required. This AC voltage must always be kept at a constant magnitude, or measured and mathematically compensated. Generating an AC voltage of constant magnitude is the to be complex in terms of circuitry and, even when using the control circuit as a microprocessor-based digital circuit, additionally requires digitization of the initially analog signal in order to be able to process it further. This is why a different solution is proposed in DE-C2-19632983.
An AC voltage regulator with adjustment to a constant RMS value is known, for example, from DE-A1-10021399. The AC voltage is adjusted by controlled phase angle control which is implemented in the form of a closed loop.
EP-A1-2154430 discloses a flame amplifier for detecting the ionization current using an ionization electrode which is disposed in the flame region of a gas burner and is connected to an AC voltage supplied by a secondary circuit of a transformer. The secondary circuit is electrically isolated from the primary circuit. In the secondary circuit, an ionization current having a DC component caused by the flame flows to an amplifier. The direct current flows through the AC voltage source to the ionization electrode and forms a closed loop with the flame. The signal processing circuit delivers a controlled variable dependent on the ionization current to a control device which compares this actual value with a setpoint value. Depending on the result, the control device generates the actuating signals for the final control elements, e.g. for a blower for adjusting the quantity of air and for a gas valve for adjusting the quantity of gas for combustion. There is no suggestion of correcting the AC voltage present at the ionization electrode as a result of line faults. Nor is attention drawn to the fact that many components, in particular the transformer, have significant tolerances and therefore systematic measurement errors occur, resulting in systematic variance in the adjusted λ-value.
WO-A1-2009/110015 discloses a method for monitoring a flame whereby parasitic elements occurring during operation can be detected and compensated. For this purpose an AC voltage source is controlled on the basis of the ionization current measured such that an AC voltage signal with markedly different duty ratio between positive and negative amplitude is generated with different amplitude values and is applied to the ionization electrode. WO-A1-2009/110015 also discloses that high AC voltages at the ionization electrode and flame and therefore also high amplitudes of the AC voltage source produce a lower dependence of the ionization signal on layers which can form on the burner and ionization electrode. Because of the nonlinear behavior of the flame, linear compensation as proposed in DE-C2-19632983 is inappropriate at the high AC voltages aimed for. The AC voltage applied must be sufficiently precise in order to eliminate systematic errors due to component variations.